This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 100 04 384.4, filed on Feb. 2, 2000, the entire disclosure of which is incorporated herein by reference.
The invention relates to an arrangement and a method for detecting and measuring strain and temperature and variations thereof, of a cover layer applied on a substrate, as well as to a method of making such a sensor arrangement. The sensor arrangement uses only a single sensor for separately determining the strain and the temperature and the variations thereof, of certain portions of surface coating layers, and achieves an application of optical sensors that is protected from outside environmental influences by being integrated into the surface coating layer.
It is becoming increasingly important to monitor, observe, control, and regulate various structural characteristics in the fields of conventional transport systems for air travel and space travel, and also in the field of motor vehicle construction. Examples of the above include: systems for determining the load realities, so-called load monitoring or load progression monitoring systems, systems for the early detection of structural damage (so-called health monitoring), systems for predictive maintenance requirements and maintenance support, for example at inaccessible locations or known critical structural locations of vehicles and equipment, and in general various xe2x80x9cadaptive systemsxe2x80x9d, among which the adaptive wing, the adaptive rotor, the adaptive pantograph, the adaptive landing gear, and the adaptive airbag are already known in the art.
In various ways, these systems contribute to satisfying the needs of the customer who has purchased the vehicle or other equipment. For example, in the case of a load progression monitoring system, the consumed operating life and the remaining operating life can be determined from the actual loads to which the vehicle or equipment has been subjected. Thereby, it becomes possible to aircraft, if the actually flown collective or total load is below the nominal collective load at which maintenance or an overhaul or taking the aircraft out of service would be required. On the other hand, such a system provides added safety, for example when the actual collective load is greater than the nominal allowable collective load, whereupon the duration of utilization would be shortened. In view of this, such a monitoring system gives an airline or other operator of aircraft the possibility of an individualized fleet management and achieves a reduction of the operating costs and particularly the inspection, maintenance and service costs.
When an aircraft, or generally any vehicle, approaches the end of its useful operating life, the effort and expense of inspection and upkeep become significantly increased. In this case the next situation arises, whereby the inspection and the like can be automated, whereby the effort and expense thereof can be reduced. Components that were originally designed and manufactured in an error-free or defect-free manner, can now be operated in a fault tolerant manner without any safety losses or limitations, which in turn will lead to an increase of the useful operating life and therewith a decrease in costs.
Furthermore, due to their own specialized adaptive capabilities, adaptive systems predominantly contribute to increases or improvements of various flight characteristics (or driving characteristics in the case of a general vehicle). Depending on the concrete application, such improvements can lead to a reduction of fuel consumption, a reduction of noise generation, an increase in speed of travel as well as safety of travel, and the like.
It is common to all of these systems that they require a very robust and reliable sensor arrangement, which ensures maximum performance with minimum hardware, effort, complexity, and expense. A pertinent quantity or parameter that can be technically measured to provide required information in such systems is especially the strain of a component or material. Conventionally, such strain is especially measured by electrical strain gages such as foil strain gages of present day technology. Alternatively, strain may be measured with various types of piezoelectric or fiber optic sensor arrangements.
Furthermore, various different concepts are known in the art, whereby optical fibers integrated into a component can be used for measuring the strain and the temperature thereof. In this context, the person of ordinary skill in the art makes use of the well known relationship or equation defining the propagation constant or coefficient xcex2 of a light wave in an optical fiber, namely: xcex2=*L, wherein n is the refractive index of the light wave, i.e. the so-called modal index, and L is the length or measuring length of the fiber. Nearly all presently known measuring concepts for measuring strain and temperature are based on the recognition that xe2x80x9cnxe2x80x9d and xe2x80x9cLxe2x80x9d in the above equation are varied as a result of strain and temperature variations. This fact also underlies the basic problem of all known temperature and strain measurements using fiber optic sensors, for all types of structural sensor arrangements, namely that the strain and the temperature have a basic and fundamental influence on the values or parameters that can be determined using available measuring technology. Thus, it is difficult or impossible to separately determine the temperature and the strain, because it is difficult or impossible to separate the influences that the temperature has on the strain, and the combined influences that the temperature and the strain both have on the measured parameters.
Published European Patent Specification EP 0,753,130 B1 discloses a system including a fiber optic Bragg grating sensor (FBGS) integrated into the structure of a fiber-reinforced composite material. A separate determination of strain and temperature is possible by means of the two polarization Bragg resonances, which arise because the optical fiber integrated into the structure becomes doubly refractive, i.e. birefringent, whereby the birefringence itself is temperature dependent. According to this reference, the temperature dependent birefringence effect is so strongly or sharply developed, that one obtains two reflection peaks from the Bragg grating. The spacing between the reflection peaks, i.e. the difference between the Bragg wavelengths of the polarization Bragg resonances, is then used as a measure for the temperature, and the Bragg wavelength of each respective peak is used as a measure for the strain and the temperature. Based on this information, a computer-supported calculation and determination of strain and temperature would be possible.
The above mentioned sensor system, however, suffers the disadvantage that it apparently only applies to fiber optic Bragg grating sensors (FBGS) integrated directly into fiber reinforced composite structural components. Thus, the sensor system must be xe2x80x9cbuilt inxe2x80x9d to the structural component as it is being fabricated. Also, the optical fiber must be ideally oriented perpendicular to the material fibers of neighboring layers. This fact has significant negative effects on the mechanical characterizing values of the structure under some circumstances, and on the effort and expense of fabrication thereof, and will not be practically acceptable to a person of ordinary skill in the art.
German Patent DE 31 42 392 C2 discloses an arrangement for a rip or crack sensor as well as an embodiment for practically realizing such an arrangement. This arrangement uses optical fibers that are integrated into a xe2x80x9cpainted-onxe2x80x9d coating layer on the surface of a substrate. This German Patent Document discloses the substrate-localized application of non-birefringent fibers on the substrate, but does not suggest the use of a fiber-optic Bragg grating, because the disclosed arrangement simply aims to detect cracks of a surface coating layer by means of the irreversible breaking or disruption of the fiber.
In practically carrying out the arrangement of the above mentioned German Patent, one first fixes the individual fiber close to a hole on a film-like or foil-like carrier. This carrier, with the fiber or fibers adhesively secured thereon, is then arranged on a substrate surface on which the fibers are positioned before they are embedded in the xe2x80x9cpainted-onxe2x80x9d coating layer that is to be monitored for the development of cracks or the like. However, due to questionable accuracy of the location and fixing of the fiber, which is to be point-wise exactly fixed on the substrate in the adhesion process using the paint or paint-on coating, it seems impossible to avoid an adhesion of the adhesive film onto the substrate while fixing the fibers. In this context, pulling-off the foil without leaving residual adhesive on the substrate can probably not be achieved. It also seems that the apparatus that is used as an auxiliary aid for fixing the fibers on the perforated film or foil, is too complicated and thus subject to malfunction or breakdown.
In view of the above, it is an object of the invention to provide an arrangement for detecting and measuring strain and temperature and their variations of a cover layer applied on a substrate, as well as a method of making such an arrangement. It is a further object of the invention to provide such an arrangement and method in which a fiber optic sensor is reliably applied onto a structure while being protected from environmental influences. The present arrangement shall be able to determine the structural strain or expansion and the structural temperature (i.e. the surface temperature) of the applied cover layer, without any special efforts or instructions. Particularly, the present arrangement shall make it possible to measure both the axial strain and the temperature of certain prescribed surface area sections using only a single sensor. The inventive sensor arrangement shall be easily applied onto a surface of essentially any structural component or material as the substrate, without requiring integration into the structure itself. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in a sensor arrangement for detecting strain and temperature of a cover layer applied on a substrate, wherein the arrangement comprises a substrate, an optical sensor that is positioned on the substrate, and a cover layer applied on the substrate and the optical sensor as well. The optical sensor comprises at least one light conducting fiber or optical fiber including a fiber core surrounded by a fiber cladding, and a Bragg grating that is irradiated or etched into the fiber in a defined section thereof. The optical fiber is either completely or at least partially embedded in and surrounded by the cover layer, whereby a force-transmitting connection exists between the cover layer, the surface of the substrate, and the optical fiber. The optical fiber is preferably directly lying in contact on the surface of the substrate. Non-circularly-symmetrical thermo-mechanical strains are induced in the fiber cladding and in the fiber core of the optical fiber due to stresses or forces that are transmitted into the optical fiber from the cover layer. As a result of these strains, the optical fiber has become birefringent.
The above described arrangement is fabricated according to the invention, by a process as follows. An adhesive carrier having a plurality of holes therein and having one adhesive surface is provided. An optical fiber is adhesively secured onto the adhesive surface of the adhesive carrier, without exceeding the allowable radius of curvature of the optical fiber, while the fiber is positioned along the row or pattern of the various hole positions of the holes in the adhesive carrier, whereby the optical fiber is aligned to extend essentially over the center of each hole, and then is adhesively fixed on the adhesive carrier in this condition. The optical fiber that has been fixed in this manner is then positioned on the surface of the substrate, and thereafter a binder material is applied to penetrate through the holes of the adhesive carrier onto the surface of the substrate so as to point-wise adhere or fix the optical fiber onto the surface of the substrate. This applied binder material is then allowed to cure, and thereafter the adhesive carrier is peeled off of the optical fiber. Then, the adhesively secured or tacked optical fiber is completely or at least partially surrounded and enclosed by a further coating medium that is surfacially applied on the surface of the substrate and on the optical fiber, so that the optical fiber becomes embedded in this applied layer of the coating medium. After this coating medium layer is cured, the optical fiber has thereby been finally secured on the surface of the substrate.
The above described arrangement is used in a method according to the invention for determining the strain and the temperature of the cover layer applied on the substrate. When a temperature dependent mechanical strain arises in the cover layer, the force transmitting connection between the cover layer and the optical fiber embedded therein transmits corresponding forces into the optical fiber and thereby induces a non-circularly-symmetrical strain in the fiber cladding and the fiber core of the optical fiber, especially in the section of the fiber in which a Bragg grating has been formed. As a result of this induced strain, the optical fiber becomes birefringent, and the resulting birefringence causes a spreading or broadening of the spectrum of the Bragg grating. By measuring the width of the spectrum of the Bragg grating, the resultant width, such as the full width at half maximum intensity (FWHM), is used as a measure of the temperature. In other words, the temperature of the coating layer can be determined from this width measure of the spectrum.
Furthermore, the sensor is preferably actuated by conducting a quasi-depolarized light into the sensor through the optical fiber. As a result of the above mentioned birefringence, which is not a very strong birefringence, the quasi-depolarized light in the Bragg grating sensor causes a total spectrum that is formed from the respective spectra of the two polarization Bragg resonances, whereby the form of this spectrum will vary preferably only as a function of temperature. The resulting full width at half maximum value is measured while providing the quasi-depolarized light as an input, and then the Bragg wavelength of this spectrum is determined as the arithmetic mean of the two individual polarization Bragg resonances. Namely, the Bragg wavelength for the spectrum resulting from the quasi-depolarized light is taken as one half of the sum of the Bragg wavelengths of the individual polarization Bragg resonances. This Bragg wavelength is used as a measure of the axial strain of the sensor as well as the temperature. Thereby, the temperature can be determined from the full width at half maximum value, and the axial strain can be determined in connection therewith from the Bragg wavelength.